It is known that, during the operation of an internal combustion engine, the rotation speed of the internal combustion engine fluctuates according to combustion cycles. Some conventional technologies detect a combustion state such as misfire on the basis of combustion cycle fluctuation of the rotation speed.
For example, according to U.S. Pat. No. 6,401,527, a combustion state is detected by comparing a rotation variation amount in the former half of one combustion cycle and a rotation variation amount in the latter half.
According to JP-2000-240500A, in the normal operation state, angular speed fluctuation difference amounts of a crankshaft within a predetermined stroke are detected plural times to obtain a probability distribution of the angular speed fluctuation difference amounts so that misfire is detected by setting a reference value for determining an abnormal combustion area on the basis of the probability distribution. In such a case, since a difference occurs in the variation amount of rotation speed of each combustion cycle between during the normal combustion and during the abnormal combustion such as misfire, a combustion state can be detected.
However, according to the conventional detection method, as an engine speed increases, the detection accuracy of combustion states decreases. Specifically, it is confirmed that the rotation speed fluctuation of each combustion cycle reduces in a high rotation range, and the reduced rotation speed fluctuation makes the differentiation between the normal combustion state and the abnormal combustion state difficult, causing reduction in the detection accuracy of combustion states.